The present invention relates to a lock device called a "Crescent Lock" and which is applied to double sliding doors.
Conventionally, this type of lock device is composed of a stationary hook fixed to one sliding door and a rotary hook fixed to another sliding door so that the rotary hook may be arranged to engage the stationary hook. When the doors are closed, the doors can be locked with the rotary hook being rotated to engage the stationary hook and the doors can be unlocked with the rotary hook being rotated in a reverse direction for disengagement from the stationary hook.
The lock device described above is usually provided with a lock-hold mechanism by which the rotation of the rotary hook is restricted. In the lock-hold mechanism a rotary shaft for the rotary hook has a projection piece which rotates with the rotary hook in one body and on the other hand a sliding type stopper having an operation unit is arranged on a setting base. The stopper is designed to rise above or retract under the surface of the rotational path when the operation unit is operated in reciprocation to allow the rotary hook to be set on or released from the stationary hook respectively.
If such a lock device is applied to double sliding doors and someone breaks a part of the glass of the door near the lock device, he can insert his hand into the room and release the lock from its lock-hold mode using the operation unit and turn the rotary hook to be unlocked opening the way for entering into the room illegally without difficulty.
The inventor first considered to provide a key-operated lock mechanism additionally to this lock device. If the locks of this type are arranged on all windows of a building, all lock-hold mechanisms may be released with a single key if such a system is designed to use a common key. In such a case, a problem will arise when the key is stolen or lost. On the contrary, if a separate kind of key is used for each lock-hold mechanism, the number of the keys becomes so numerous that their control is naturally a troublesome problem.